Proximity detection system

ABSTRACT

Proximity sensing arrangements utilize one or more sensing devices to detect and/or to discern objects that are or become proximate, or are expected to be proximate to a position of interest. For example, one or more capacitive sensors may be distributed to locations about a machine, e.g., attached to, incorporated with, or otherwise associated with a moving or otherwise operating component of the machine. Corresponding control electronics drive each capacitive sensor, using a corresponding excitation signal, as well as process information read from each capacitive sensor to make intelligent proximity related decisions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/943,630, filed Nov. 21, 2007, entitled “PROXIMITY DETECTION SYSTEM”,now allowed, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/867,514 filed Nov. 28, 2006, entitled “PROXIMITYSENSING SYSTEM”, the disclosures of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to systems, computer program products andmethods for proximity sensing, such as may be utilized for collisionavoidance, obstacle detection and other tasks where it is desirable todetect, track, monitor or otherwise recognize either the presence ofobjects that are or become proximate to a position of interest or theabsence of objects proximate to the position of interest.

Many systems comprising automated and semi-automated machines, transportmechanisms, robotic devices, tools and other industrial devices includeone or more components that move or otherwise operate within a definedspace. The operation of such components within their defined space istypically initiated to perform work corresponding to a predeterminedtask and may be programmably automated, autonomously implemented,manually controlled or directed, or otherwise determined by theparticular task being implemented. However, objects in the same space asthe moving or otherwise operating component may cause unintentionalcontact with the component. Further, under certain circumstances, even astationary component of a system may be exposed to the potential ofunintentional contact with objects moving within the space of thestationary component.

BRIEF SUMMARY OF THE INVENTION

According to aspects of the present invention, a proximity detectionsystem comprises an excitation source that generates an excitationsignal, a capacitive sensor, a monitor, a mapping and a controller. Thecapacitive sensor has a sense electrode that secures to a surface of asystem component and forms a circuit with the excitation source togenerate a capacitive field about the sense electrode at least in adirection of interest, wherein the capacitive field changes due to thepresence of objects present within the capacitive field, thus changingthe overall capacitance of the capacitive sensor. The monitor is coupledto the circuit formed of the excitation source and capacitive sensor andobtains measurements that are influenced by the capacitive fieldassociated with the capacitive sensor. The mapping comprises a mappingof threshold values at a plurality of positions of the capacitive sensorwithin a space. The controller, for a given position of the capacitivesensor within the mapped space, determines a measured value from atleast one measurement from the monitor for the given position,retrieves, based at least in part upon the given position, a thresholdvalue from the mapping to derive an anticipated value, performs anevaluation based upon the determined value and the anticipated value andperforms a predetermined action if the determined value is outside apredetermined range of the anticipated value.

According to further aspects of the present invention, a proximitydetection system comprises an excitation source that generates anexcitation signal, a capacitive sensor, a monitor, a mapping and acontroller. The capacitive sensor has a sense electrode that secures toa surface of a system component and forms a circuit with the excitationsource to generate a capacitive field about the sense electrode at leastin a direction of interest, wherein the capacitive field changes due tothe presence of objects present within the capacitive field, thuschanging the overall capacitance of the capacitive sensor. The monitoris coupled to the circuit formed of the excitation source and capacitivesensor and obtains measurements that are influenced by the capacitivefield associated with the capacitive sensor. The mapping comprises amapping of threshold values at a plurality of points in time relative toa start time of a programmed operation of the system component to whichthe capacitive sensor is attached, each threshold value uniquelyassociated with the corresponding point in time of the correspondingprogrammed operation. The controller evaluates the capacitive field ofthe capacitive sensor during execution of the programmed operation ofthe system component at points in time corresponding to the mapping ofthreshold values, wherein the controller, for a given point in time ofthe mapping, determines a measured value from at least one measurementfrom the monitor, retrieves a threshold value from the mappingcorresponding to the given point in time of the mapping, to derive ananticipated value, performs an evaluation based upon the determinedvalue and the anticipated value and performs a predetermined action ifthe determined value is outside a predetermined range of the anticipatedvalue.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of various embodiments of the presentinvention can be best understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals, and in which:

FIG. 1 is a block diagram of an exemplary capacitive sensor arrangementaccording to at least one aspect of the present invention;

FIG. 2 is a schematic block diagram illustrating an exemplaryarrangement of the control electronics that may be utilized with thecapacitive sensor of FIG. 1;

FIG. 3 is a schematic block diagram illustrating an exemplaryimplementation of the processing of proximity information read by thecontrol electronics such as may be implemented for example, by thedigital signal processor shown in FIG. 2;

FIG. 4 is an illustration of an exemplary excitation signal showingseveral exemplary sample points;

FIG. 5 is a schematic block diagram illustrating an exemplaryimplementation of the processing of proximity information read bycorresponding control electronics;

FIG. 6 is a schematic illustration of a threshold table for trackingthreshold values across a range of positions according to an aspect ofthe present invention;

FIG. 7 is a schematic illustration of a threshold table for trackingthreshold values across a range of positions according to another aspectof the present invention;

FIG. 8 is a schematic block diagram illustrating the use ofelectrodes/shields external to a sensor to discriminate objects that areanticipated to be proximate to the sensor and which are not to bedetected;

FIG. 9 illustrates an exemplary gear inspection machine that mayincorporate various aspects of the sensor systems of FIGS. 1-8;

FIG. 10 is an illustration of a system having multiple sensors; and

FIG. 11 is a cross-section illustration of a three dimensional sensorassembly that is assembled on a rectangular system component forpurposes of illustration.

DETAILED DESCRIPTION OF THE INVENTION

The various aspects of the present invention may be embodied as systems,methods including computer-implemented methods, and/or computer programproducts. Also, various aspects of the present invention may take theform of an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware, wherein the embodiment oraspects thereof may be generally referred to as a “circuit,” “module” or“system.” Furthermore, various aspects of the present invention may takethe form of a computer program product on a computer-usable storagemedium having computer-usable program code embodied in the medium.

In the detailed description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration, and not by way of limitation, specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and that changes may be made withoutdeparting from the spirit and scope of various embodiments of thepresent invention.

Referring now to the drawings and particularly to FIG. 1, a proximitydetection system 10 is illustrated, which can detect the presence ofobjects 12 in proximity to one or more positions of interest in adefined space. In a typical application, the proximity detection system10 may be implemented as part of, or otherwise integrated with, a largersystem, such as to monitor positions of interest to a machine 14 forcollision avoidance, obstacle detection and/or other tasks such as whereit may be desirable to detect, track, monitor or otherwise recognizeeither the presence of objects that are or become proximate to aposition of interest or the absence of objects proximate to the positionof interest.

The term “system” is intended to be considered in broad and generalterms herein, so as to include, for example, one or more automated orsemi-automated machines such as inspection machines,processing/finishing/handling machines, milling and other cuttingmachines, a machine sub-assembly, a group of related or otherwiseinteracting machines, one or more transport mechanisms, robots and otherrobotic devices, tools and/or industrial devices or any otherarrangement where proximity sensing is desired, such as to prevent orotherwise mitigate proximity-based or contact-based damage to structuresof the system.

The proximity detection system 10 comprises, in general, at least onecapacitive sensor 16 coupled to corresponding control electronics 18.Each capacitive sensor 16 is utilized to designate a position ofinterest within a defined space to the machine 14, and may be coupled toa surface of a system component 14A, such as a movable machine part or amachine part that might otherwise potentially contact unintended objectswithin a defined space in which the part is located. As will bedescribed in greater detail below, the presence (or absence) of objectsthat are of interest in detecting may include machinery, tooling, a workpiece, environmental structures, people, etc. that are or becomeproximate to the capacitive sensor 16 and correspondingly, the systemcomponent 14A that the capacitive sensor 16 is attached to. Asillustrated, each capacitive sensor 16 comprises a sensor plate, alsoreferred to herein as a sense electrode 20, that is spaced from thecorresponding system component 14A by a shielding plate, which is alsoreferred to herein as a shield electrode 22.

The control electronics 18 contains circuitry necessary to generate anelectric field about the sense electrode 20. The control electronics 18drives the shield electrode 22 coincident with the sense electrode 20 sothat the generated electric field is reflected away from the machine 14proximate to the sense electrode 20, which extends the range of thecapacitive sensor 16 in the direction(s) of interest. The controlelectronics 18 also measures the field loading caused by objects 12moved into the field proximate to the sense electrode 20. Still further,the control electronics 18 may analyze the field loading and trigger theappropriate response, or the control electronics 18 may pass themeasured information along to another processor (not shown) for analysisand/or action.

More particularly, the sense electrode 20 acts as one plate of acapacitor 24. As such, for purposes of illustration, a capacitor isschematically illustrated in FIG. 1 in phantom lines. The other platerequired to complete the capacitor 24 is defined by any grounded object12 that is in, or moves into, proximity with the sense electrode 20 thathas a dielectric constant different from air in the exemplaryimplementation. Essentially, as the object 12 approaches the vicinity ofthe sense electrode 20, the electric field around the capacitive sensor16 is altered. This change in the electric field is detected by thecontrol electronics 18. The control electronics 18 may then perform apredetermined operation as the specific application dictates. Thus, anapproach may be devised for the system, e.g., the machine 14, to addressunexpected or unanticipated events by detecting, tracking, monitoring orotherwise recognizing the proximity (or absence of proximity) of objects12 in space with reference to the capacitive sensor 16.

For example, a controller, which may be provided as part of the controlelectronics 18, may be operatively configured to evaluate one or moremeasurements that are influenced by the capacitive field associated withthe capacitive sensor 16 and to trigger a predetermined action if theevaluation corresponds to a triggering event. In this regard, thecontroller may perform a comparison of a threshold value to ameasurement, perform a calculation or otherwise perform a manipulationof information related to the capacitance of the capacitor 24. Thecontroller may further trigger a predetermined action, e.g., output acontrol signal or message to a host application or otherwise takenecessary action, e.g., to implement a collision avoidance maneuver, toperform other remediation or take other action if the comparisoncorresponds to a triggering event. Thus, for example, if an unexpectedobject is or becomes proximate to the capacitive sensor 16 such that themeasured capacitance exceeds a threshold value or otherwise causes someother defined condition(s) to be satisfied, an appropriate correctiveaction may be taken, such as to avoid contact between a system component14A and a corresponding object within the defined space of the part.

As another example, the capacitive sensor 16 may be used to identify thelack of presence (absence) of an anticipated or expected object. In thisregard, assume that the capacitive sensor 16 is secured to an arm of aninspection machine. As the arm approaches a work piece to be inspected,it is anticipated that the measured field will change as a direct resultof the proximity of the work piece. However, if the field does notchange by an appropriate amount at a position of the arm where a changein the capacitive measurement is expected, then a predetermined actionmay be triggered, such as a warning or message that the work piece to beinspected is missing.

As yet another example, the capacitive sensor 16 may be used to identifythe detection of an anticipated or expected object. In this regard,assume that the capacitive sensor 16 is secured to an arm of aninspection machine. As the arm approaches a work piece to be inspected,it is anticipated that the measured field will change as a direct resultof the proximity of the work piece. If the field appropriately changesat a position of the arm where a corresponding change in the capacitivemeasurement is expected, e.g., based upon a predetermined signature ofcapacitive field as modified by the expected work piece, then aninference may be drawn that the proper part is available for inspection,which may trigger a next step in an inspection workflow.

Correspondingly, under certain circumstances, the capacitive sensor 16may even be able to tell that a wrong part has been presented forinspection (or in the general case, when a wrong object of interest hasbeen detected). For example, if the measured capacitance is higher orlower than what is expected, e.g., where the wrong part/object has adifferent dielectric constant than the expected part or otherwisedisrupts the field of capacitive sensor 16 in an unexpected way, anappropriate action can be implemented.

As yet a further example, one or more sensors 16 can be used to locateobjects, and/or position a part of a machine proximate to an object thatis at a known or unknown location, such as by scanning a bound volume ofspace for changes in the field of the capacitive sensor 16.

Referring to FIG. 2, an exemplary implementation of the controlelectronics 18 comprises in general, an excitation source 32 and acircuit 34 that couples the excitation source 32 to the capacitivesensor 16. The control electronics 18 also comprise a monitor 36 and acontroller 38. The excitation source 32 provides an excitation signalthat drives the circuit 34 and correspondingly, the capacitive sensor16. The monitor 36 performs measurements of the circuit where themeasurements are influenced by the capacitive field associated with thecapacitive sensor 16 and the controller 38 evaluates one or moremeasurements taken by the monitor and triggers a predetermined action ifthe evaluation corresponds to a triggering event as will be described ingreater detail herein.

In an exemplary implementation, the excitation source 32 comprises adigital signal controller 38 having one or more signal lines that arecoupled to a digital to analog converter 40. The output of the digitalto analog converter 40 may further be coupled to a signal conditioningcircuit 42. The output of the signal conditioning circuit 42 is theexcitation signal used to drive the circuit 34. The controller 38, e.g.,a suitable microcontroller or other digital signal processing deviceexecutes code to output a digital representation of the desiredexcitation signal. For example, the controller 38 may implement suitablesoftware code to generate a periodic signal such as a sine wave, squarewave, triangle wave, sawtooth, etc., by retrieving data values from anM-point lookup table that is stored in a memory accessible by thecontroller 38. The M-points may collectively describe points thatapproximate a single cycle of the desired excitation signal. The digitalrepresentation of the excitation signal output from the digital signalprocessor thus comprises an M-step approximation of the excitationsignal and may be provided as a pulse-width modulated output, a streamof multi-bit words, or in any other suitable format, e.g., as iscompatible with the technology utilized to implement the digital toanalog converter 40.

The digital to analog converter 40 converts the digital representationof the excitation signal to an analog representation of the excitationsignal. The signal conditioning circuit 42 comprises circuitry necessaryto properly condition the analog excitation signal and/or drive thenetwork 34, such as by implementing any combination of buffers,impedance converters, amplifiers, filters, etc. For example, the signalconditioning circuit 42 may smooth the output of the digital to analogconverter 40 using a low pass filter, then buffer and/or amplify thesignal using an amplifier, e.g., a non-inverting, zero phase shift,unity gain amplifier. The output of the amplifier is used to drive thenetwork 34, including the sense electrode 20 (shown in FIG. 2 as asensor plate for purposes of illustration) of the capacitive sensor 16.

Although one illustrative implementation of the excitation source 32 isschematically shown in FIG. 2, other arrangements may be suitablyprovided to generate the excitation signal, including the use of analogcircuitry, such as an analog oscillator circuit. Further, alternativedigital, analog and/or combinations of digital and analogimplementations may also be used. Also, the various components of theexcitation source 32 may be integrated into one or more physicalcomponents, e.g., an application specific integrated circuit (ASIC),microcontroller, reconfigurable device, such as a field programmablegate array (FPGA), programmable gate array (PGA), Programmable LogicArrays (PLA), programmable oscillator, direct digital synthesis chipand/or other necessary logic. Also, as will be described in greaterdetail herein, the shape of the excitation signal may be relativelyunimportant if subsequent narrow band filtering is used to rejectundesired harmonics that may be present.

In an exemplary implementation, the excitation signal comprises aperiodic signal and the circuit 34 comprises a resistance componentschematically shown as resistor 44 coupled to the capacitive sensor 16.As another example, the circuit 34 may comprise a voltage divider/filtercircuit such as using resistance and capacitance components. Theimplementation of any desired low pass filtering, e.g., within thecircuit 34 or the signal conditioning circuit 42 may depend, forexample, upon system specifications, such as the selected excitationfrequency f or anticipated range of frequencies over which theexcitation signal may be operated.

A sensor cable 46 is utilized to couple the resistor 44 to thecapacitive sensor 16. For example, as illustrated, the excitation signalis communicated between the resistor 44 and the sensor plate 20 of thecapacitive sensor 16 via a first conductor 46A of the sensor cable 46. Asecond conductor 46B of the sensor cable 46 provides a return from thesensor plate 20 to the monitor 36.

The total capacitance that is measured by the monitor 36 comprises thecapacitance that corresponds to the field created with respect to thesense electrode 20, i.e., the capacitance of the capacitor 24 formed inpart, by the sense electrode 20, as will be explained in greater detailherein. Other implementations of the circuit 34 may be utilized so longas changes to the field associated with the sense electrode 20 can bedetermined.

For maximum sensitivity to small changes in sensor capacitance, theresistance of the resistor 44 may be matched to the anticipated nominalcapacitance. In an exemplary implementation, the magnitude of the gainof the circuit 34 is given by:

${G} = \frac{1}{\sqrt{1 + \left( {2 \cdot \pi \cdot f \cdot r \cdot c} \right)^{2}}}$

To find the resistance that causes the maximum rate of change of gainwith respect to capacitance, the second derivative of the aboveexpression may be set to zero:

${\frac{\mathbb{d}^{2}}{\mathbb{d}c^{2}}\frac{1}{\sqrt{1 + \left( {2 \cdot \pi \cdot f \cdot r \cdot c} \right)^{2}}}} = {{4 \cdot \pi^{2} \cdot f^{2} \cdot r^{2} \cdot \frac{\left( {{8 \cdot \pi^{2} \cdot f^{2} \cdot r^{2} \cdot c^{2}} - 1} \right)}{\left( {1 + {4 \cdot \pi^{2} \cdot f^{2} \cdot r^{2} \cdot c^{2}}} \right)^{\frac{5}{2}}}} = 0}$

This results in the following condition:

${2 \cdot \pi \cdot f \cdot r \cdot c} = \frac{1}{\sqrt{2}}$

If this condition is met, the voltage divider gain will be approximately0.816. Therefore, to find the correct value of resistance, r is set tozero and the resulting discrete Fourier transform reading is recorded.Then r is increased until the reading drops to 0.8 times the originalreading. This is the resistance to use for maximum sensitivity. Thefirst derivative of the gain gives the sensitivity:

${\frac{\mathbb{d}}{\mathbb{d}c}\frac{1}{\sqrt{1 + \left( {2 \cdot \pi \cdot f \cdot r \cdot c} \right)^{2}}}} = {\frac{- 4}{\left( {1 + {4 \cdot \pi^{2} \cdot f^{2} \cdot r^{2} \cdot c^{2}}} \right)^{\frac{3}{2}}} \cdot \pi^{2} \cdot f^{2} \cdot r^{2} \cdot c}$

If the optimizing condition is substituted into the above expression,the sensitivity becomes:

$\frac{- 1}{2 \cdot c \cdot (1.5)^{1.5}} = \frac{- {.272}}{c}$

This shows that the magnitude of the sensitivity can be maximized byminimizing the ambient capacitance, c. The capacitance can be minimizedby keeping sensor cable 46 as short as possible and by utilizing activeshielding.

In an illustrative example, the excitation signal comprises a constantamplitude periodic signal having a constant frequency f. Accordingly,the amplitude of the voltage at frequency f is a measure of the totalcapacitance of the network 34, including the capacitance associated withthe field about the sense electrode 20 of a corresponding capacitivesensor 16. If two or more sensors 16 operate in the same environment,e.g., on different component surfaces of the same machine, then eachcapacitive sensor 16 can be driven at a different excitation frequencyto prevent interference from the electric fields generated by nearbysensors 16. Under this arrangement, the frequency of a first capacitivesensor 16 may be chosen so that no harmonics fall within the sensitiverange of another capacitive sensor 16. Moreover, band filtering ofharmonic content and/or other signal processing of the measuredexcitation signals can relax the requirements for different excitationsignals. The implementation of multiple sensors is described in greaterdetail herein.

In the illustrated system, the monitor 36 senses the capacitance of thecircuit 44 by measuring the amplitude of the voltage that appears at thesensor plate, i.e., sense electrode 20 at point A. In this regard, themonitor 36 comprises a first buffer/amplifier 48 that buffers and/oramplifies the signal from point A. The impedance at node A is likelyrelatively high, so the buffer/amplifier 48, e.g., an instrumentationamplifier, may be provided to isolate node A from other circuitry. Theoutput of the first buffer/amplifier 48 is coupled to an analog todigital converter 50, which outputs a digital representation of theoutput of the first buffer/amplifier to the controller 38. In anexemplary implementation of the processing electronics 18, the analog todigital converter 50 is synchronized to the corresponding digital toanalog converter 40, as will be described in greater detail below.

A copy of the input to the analog to digital converter 50 is optionallyscaled, such as by a compensation circuit, e.g., a voltage divider (notshown) and the optionally scaled signal is buffered by a driver circuit54, e.g., as implemented by a shield driver amplifier and optionaladditional circuitry as required. The output of the driver circuit 54 isutilized to drive the shield electrode 22 of the capacitive sensor 16via a third conductor of the sensor cable 46. Further, a fourthconductor of the sensor cable 46 provides a feedback signal from theshield plate, i.e., shield electrode 22 back to an input of the drivercircuit 54. The driver circuit 54 may alternatively be replaced with anysuitable amplifier, signal conditioning and scaling circuitry.

Remote sensing is used in both the first buffer/amplifier 48, i.e., thesignal amplifier and the driver circuit 54, such as the illustratedshield driver amplifier to reduce the effects of cable impedance. Also,as shown, both amplifiers are connected as voltage followers. However,gain scaling, impedance conversion and other additional processing maybe implemented based upon the specific requirements. As an example, thefirst buffer/amplifier 48 (signal amplifier) may comprise a type ofoperational amplifier that has a low input offset current to minimizethe loading of the sensor circuit by the amplifier and the drivercircuit 54 (shield amplifier) may comprise a type of operationalamplifier that can drive high capacitive loads.

As noted above, the capacitance of the circuit is influenced by thecapacitive field generated by the capacitive sensor. In this regard, thefield that is being measured is the field that is being generated, e.g.,via the excitation signal. It is not some undefined, random field thatis being measured.

As noted above, the output of the analog to digital converter 50 is fedback to the controller 38 for further processing. As an example, thecontroller 38 may compute the Discrete Fourier Transform (DFT)coefficient of the signal fed back from the analog to digital converter50 at the excitation frequency f used to drive the network 34. The DFTcoefficient is a complex number that describes the magnitude as well asthe phase angle of the signal measured at node A, taken at theexcitation frequency f. By limiting the DFT computation to the knownexcitation frequency f, interference at other frequencies may beminimized and computational overhead of implementing a more complex DFTalgorithm is avoided. However, other filtering and analyzing processesmay be performed. For example, as will be described in greater detailherein, what is referred to as a DFT filter herein may be considered ademodulator because its output is not a filtered version of its inputstream. Instead, the output is a measure of the amplitude of the signalat the tuned frequency. If the sample size, referred to as ‘M’, ischosen appropriately, then the phase angle of the signal can becomputed. If this is the case, a DFT demodulator could be used as a verysensitive, fast, and low-noise LVDT signal conditioner. Also, the DFTmay be replaced by a bandpass filter (analog or digital) or othersuitable filter(s).

The above process is periodically repeated, e.g., whenever one completecycle of the excitation signal has been generated. Moreover, the rate atwhich the DFT coefficients are processed may be reduced, such as byfiltering the DFT coefficient stream with a decimating low pass filter.The additional filtering and processing may be performed, for example,within the controller 38.

Other suitable techniques may alternatively be utilized to implement thecontrol electronics 18. For example, the capacitive sensor 16 may becoupled to a tuned oscillator circuit. Under this arrangement, changesin the capacitance between the sense shield 20 and the object 12 alterthe frequency of a tuned oscillator circuit. The amount of capacitancechange corresponds to a change in the frequency of the oscillator. Stillfurther techniques may alternatively be utilized to detect changes incapacitance of the capacitive sensor 16.

Referring to FIG. 3, an example of processing that may be performed bythe monitor 36 is illustrated. A circular buffer 60, e.g., within thecontroller 38, holds N measurements, e.g., digitized voltage readings,where N is any integer greater that or equal to M and M is the number ofpoints in one cycle of the excitation signal generated by the excitationsource 32. An exemplary approach to filling the circular buffer 60 is byconstantly accumulating measurements or readings representing thesampled data from the analog to digital converter 50 in groups of sizeM. These readings may be saved in an alternating fashion, e.g., to aselect one of two sample buffers 62, 64, either Buffer A or Buffer B asshown. When the currently selected sample buffer, e.g., Buffer A,becomes filled with M readings, its contents are transferred to thecircular buffer 60 and the oldest samples in the circular buffer 60 arediscarded. Also, the output of the analog to digital converter 50 isswitched so as to accumulate readings into the remaining sample buffer,e.g., Buffer B in the above example.

After a group of M samples has been transferred into the circular buffer60, the real and imaginary (cosine and sine) components of the DFT arecalculated by a DFT cosine component filter at 66 and a DFT sinecomponent filter at 68 respectively, for the entire contents of thecircular buffer 60. These real and imaginary components, designated ‘C’and ‘S’ in FIG. 3, are combined at 70, e.g., by taking the square rootof the sum of C squared and S squared. The samples output at 70,designated ‘D’ may be passed on to a decimating low pass filter 72 thatreduces the noise level and also reduces the sample rate to a manageablelevel if required by the particular implementation. The calculations at66, 68 and 70, and the implementation of the low pass filter 72 (ifrequired) may be performed in software, for example, as executed by thecontroller 38 using suitable algorithms and techniques.

The samples, D, coming from the output at 70 (or low pass filter 72) maybe analyzed to identify events or conditions of interest. For example,the samples D may be compared to a pre-determined threshold value at 74and an action may be triggered if the comparison corresponds to atriggering event. Thus, in an illustrative example, if the value of thesample D is less than the threshold, then the sensor capacitance isgreater than the corresponding threshold capacitance, which may indicatethe proximity of an object.

In this regard, it is observed from the schematic illustration shown inFIG. 2 that the measurement is taken at node A, at the junction of thecapacitor 24, via the sense electrode 20 and the resistance component44. The capacitive reactance of the capacitor 24 decreases as a functionof increasing frequency thus forming an AC voltage divider or low passfilter circuit that discriminates against high frequencies. That is:

${{Vout}} = {{{Vin}}\frac{1}{\sqrt{\left( {1 + {\omega^{2}R^{2}C^{2}}} \right)}}}$

If an object interferes with the field, the total capacitance (C) of thecapacitor 24 increases. Thus, for a given excitation signal frequency (fwhere ω=2πf) and amplitude |Vin| and for a given resistance component 44(R), an increase in the total capacitance C causes a decrease in |Vout|.In this regard, |Vout| corresponds to the measurements and thus isrelated to the computed value D.

Accordingly, an action can be taken at 76, e.g., implementing acollision avoidance operation or performing any other desired action.The action taken at 76 may alternatively consider numerous other and/oralternative factors to a comparison with a threshold value. For example,an action may be based upon, or further consider, detected changes inthe sample value D, the time rate of change of the samples D, or otherfactors that define appropriate action criteria.

Although an illustrative and exemplary system is shown and describedthat utilizes two sample buffers 62, 64 and a corresponding circularbuffer 60, other arrangements may be implemented to analyze the outputfrom the analog to digital converter 50. Further, it may be possible toperform simplifications, such as computing only one of the DFTcomponents, depending upon the requirements of a specific implementationof the controller 38.

In an illustrative implementation, the controller 38 is configured basedupon a plurality of assignable parameters. This allows, for example,easy changing of system performance, simply by changing one or more ofthe parameters. Some exemplary programmable parameters that are usefulto set up a capacitive sensor 16 comprises an excitation frequency f,the number of cycles K (which may be implied if only a single cycle isutilized), a filter output sample rate fsr and a DFT filter length N.

The number of cycles K may be used to define the number of cyclesrepresented in the lookup table data that describes the excitationsignal and may be set to a value of 1 or implied as having a value of 1,for example, in a system with single cycle excitation signal lookuptable. However, there are instances where it is useful to store amulti-cycle approximation of the excitation signal in a lookup table.For example, the digital to analog converter 40 and the analog todigital converter 50 may be replaced with a single codec. However, suchdevices typically have a limited number of sample rates, e.g., 48 kHzand 96 kHz.

Thus, depending upon the algorithm used to implement the digitalrepresentation of the excitation signal, a multi-cycle waveapproximation can be stored in memory as an alternative to a singlecycle wave approximation to digitally describe the excitation signal.The modified approach approximates a wave over several cycles, asschematically represented in FIG. 4.

Using the values stored or otherwise programmed into these parameters,the controller 38 may calculate the data lookup values for a T lengthperiodic wave look up table. For example, an integer number ofexcitation cycles, K of a periodic signal, e.g., a sine wave is chosenand the wave is approximated over that number of cycles. The totalnumber of steps, T, in the periodic wave approximation is given by:

$T = {{int}\left( {K \cdot \frac{sr}{f}} \right)}$

where sr is the known digital to analog conversion sample rate. The Tscaled steps may be placed, for example, in a lookup table and may beutilized by the excitation source 32 (shown in FIG. 2) to generate theexcitation signal that is used to drive the circuit 34 as described ingreater detail herein. The maximum generated frequency f should be lessthan one half of the sample rate to satisfy the Nyquist condition.

Two N length coefficient buffers are also calculated, one for the DFTcosine component filter 66 and one for the DFT sine component filter 68.

The angular spacing between points in the D/A sine wave approximation,Δa, is:

${\Delta\; a} = {2 \cdot \pi \cdot \frac{K}{T}}$

This angular difference is used in the construction of the filtercoefficient buffers, so the filters are automatically tuned to theexcitation frequency. The coefficient buffers may be modified by awindowing curve or other suitable processing techniques to improve thesignal to noise ratio.

Referring to FIG. 5, the illustrated processing system is similar tothat described with reference to FIG. 3, but is altered in a manner totake into consideration, the multi-cycle approximation of the excitationfrequency. Assume that the analog to digital converter is operated atthe same sample rate as the digital to analog converter, and storesreadings in groups of size M in one of two “ping-pong” sample buffers, Aand B, also designated in FIG. 5 as 62, 64 respectively. The group sizeM may be set, for example, by the desired filter output sample rate:

$M = {{int}\left( \frac{sr}{fsr} \right)}$

After a select one of the sample buffers 62, 64 is filled, e.g., BufferA, its contents are transferred to the N length circular buffer 60(where N is greater than or equal to M) and the M oldest readings in thecircular buffer 60 are discarded. At this time, the sine and cosine DFTfilter outputs S and C are computed at 66 and 68 respectively, forexample, according to the following equations:

$C = {\sum\limits_{i = 1}^{N}\left( {{cc}_{i} \cdot r_{i}} \right)}$$S = {\sum\limits_{i = 1}^{N}\left( {{sc}_{i} \cdot r_{i}} \right)}$

where cc_(i) is a cosine coefficient, sc_(i) is a sine coefficient, andr_(i) is a circular buffer reading. The final result is computed bytaking the square root of the sum of the squares of S and C at 70. Thatis:D=√{square root over (S ² +C ²)}

Calculating the filter results after a group of readings have been takenmay be utilized to reduce the filter output sample rate to a moremanageable level and to provide more time for the filter subroutines torun, if necessary or desirable in the particular implementation.

The filter response time, ft, is determined by the length of the filter,N:

${ft} = \frac{N}{2 \cdot {sr}}$

In a manner analogous to that described with reference to FIG. 3, thesamples, D, coming from process at 70 (or the optional low pass filter72) may be analyzed to identify events or conditions of interest. Forexample, the sample D may be compared to a pre-determined thresholdvalue at 74. If the value of the sample D is less than the thresholdthen the sensor capacitance is greater than the threshold capacitance,indicating the proximity of an object in the present example.Accordingly, an action can be taken at 76, e.g., implementing acollision avoidance operation or performing any other desired action.

Depending upon the intended application, it is possible that the ambientcapacitance is likely to change as the capacitive sensor 16 is movedabout a defined space. If this is the case, it may be necessary tomodify the criteria upon which the data measured by the monitor 36 isanalyzed, thus providing spatial accommodations for anticipatedenvironmental conditions. For example, the threshold value used for adecision at 74 may need to be compensated, adjusted or otherwisemodified for different positions in space.

In one exemplary use, the capacitive sensor 16 may be coupled to an armor other movable part of the machine 14, which moves about in a definedspace. If the application of the capacitive sensor 16 is to providecollision avoidance, the signal measured by the monitor 36 may becompared to a threshold to designate the likelihood of an imminentcollision. However, at different points in space and/or at differentpoints in time, the measured capacitance of the capacitive sensor 16 mayvary, even with no threat of collision, e.g., under steady-state ambientconditions. This may be due, for example, to structures that interferewith the field of the capacitive sensor 16, which are incapable ofcausing a collision, such as the proximity of the capacitive sensor 16to the housing or other fixed or movable components of a correspondingmachine or other surfaces associated with a given system.

According to an aspect of the present invention, the control electronicsmay integrate intelligent apparatus spatial data with the proximitysensor data read from each capacitive sensor for dynamic thresholdcalibration and/or run-time threshold adjustment of the capacitor sensoroutput. This allows, for example, the system 10 to ignore the known orpredicted intrusion of objects proximate to a corresponding capacitivesensor 16 that will not cause a collision. Thus, as will be described ingreater detail below, components that are programmed to transition orare otherwise manually moved into close proximity with, but will not orcannot collide with the capacitive sensor/system component, can be“programmed out” so as to not trigger a collision avoidance scheme. Themapping may be implemented as a function of detected, measured,approximated or otherwise known position, time, condition or otherparameter. Moreover, the mapping may comprise a single ormultidimensional range of values, offsets or other parameters.

Referring to FIG. 6, the threshold value or other criteria used toevaluate the data measured by the monitor 36, e.g., the comparison ofsamples D to threshold values at 74 and corresponding action at 76, maybe dynamically updated, modified, compensated, or otherwise adjusted ina manner that considers the anticipated actual ambient capacitance atvarious points in space and/or the ambient capacitance at positions overtime. For example, an envelope of the capacitive sensor 16 may bedivided into an array of blocks 82. The array of blocks 82 may be asingle dimension or multidimensional, and may further include acorresponding identification, e.g., each block 82 may be assigned anidentifying number. A program, which may be executed in its own threadin the controller 38, a motion controller 84 or other processing device,monitors the position of the capacitive sensor 16 or componentassociated with the capacitive sensor 16, and sends the current blocknumber, k, to the controller 38.

The controller 38 uses the received information K to select anappropriate block number or other corresponding identifier. In anexemplary implementation, the block number K serves as an index into atable of threshold values defined by the array of blocks 82 to retrievethe appropriate threshold or other information, e.g., as seen by thecurrent threshold block 82A, for use in determining whether an action isrequired. In practice, the threshold values and/or other information fordynamic updating may be stored in one or more tables, databases, arrays,linked lists, blocks, segments or other logical arrangements that allowthe controller 38 to retrieve the appropriate information.

For example, when the capacitive sensor 16 moves in space, its positioncan be mapped from one block to another. In addition or as analternative, the controller 38 may utilize the known position of anexternal object or objects, e.g., another moving or stationary surfaceof the system relative to the sense electrode 20, to select anappropriate index into the table(s) 82. As noted above, the positioninformation provided by the motion controller 84 may map to staticblocks in the array of blocks 82, such as where the desired thresholdfor a given point in the defined space does not change over time.Alternatively, the mapping of a point in the defined space to acorresponding block in the array of blocks 82 can vary dynamically,e.g., over a prescribed interval, etc. Accordingly, the controller 38can accommodate not only preprogrammed or otherwise anticipatedtransitions, but also approved transitions that may occur asynchronouslyduring an operation.

The threshold table(s) 82 may be built, for example, by a mappingprocedure in which the capacitive sensor 16 is moved throughout itsrange of motion within a defined space. The defined space is furtherconceptually mapped out into blocks 82 and sensor capacitance readingsare recorded for each block 82. In this regard, the number and/or sizeof the blocks 82 may be utilized, for example, to determine thesensitivity of the system. Moreover, the blocks can be of varying sizesand shapes within the predefined space, so long as the system canuniquely identify the location of the capacitive sensor 16 as beingpositioned in a select one of the blocks 82. That is, the pointsrecorded in the blocks 82 need not be evenly distributed throughout thedefined space. Rather, the “resolution” or granularity of the definedspace may be increased or decreased at areas or volumes throughout thedefined space, e.g., as defined by more blocks 82 of threshold data perunit of the defined space in selected regions. Thus, the granularity mayvary in one or more dimensions in any desired manner that is appropriatefor the given application.

Moreover, the table(s) 82 can be multi-dimensional not only in spacedimensions, e.g., X, Y and Z coordinates, but also in other dimensions.This aspect allows the proximity detection system 10 to take intoconsideration, multiple alternative threshold values at a given point ofthe capacitive sensor 16 in space, e.g., to account for a dynamicenvironment. For example, at a given point in space, a system may definemultiple permutations of likely or otherwise anticipated objects thatmay or may not be proximate to the capacitive sensor 16. Thus, during apredetermined period of time, known objects may advance towards, thenretreat from the known position of the capacitive sensor 16, which maybe at a fixed position. Even though the capacitive sensor 16 is notmoving, the threshold value is updated to account for the known objectthat is performing permitted movements about the space of the capacitivesensor 16. Still further, a separate table may be utilized for eachdimension, or other techniques may be utilizes to store the information.

As an example of mapping, the mapping space may be defined by assigninglower and upper limits to axis positions. For example, a point (x,y,z)may be determined to be within the mapping space if:

-   -   Xmin≦x<Xmax, Ymin≦y<Ymax, and Zmin≦z<Zmax

Under this arrangement, Xmin and Xmax are the minimum and maximum limitsfor the x axis, Ymin and Ymax are the minimum and maximum limits for they axis and Zmin and Zmax are the minimum and maximum limits for the zaxis. Points within the mapped space are defined by normalizedcoordinates, such as positive integers ranging from zero to somepredetermined maximum value. For example, if the desired axis range isfrom 0 to 255, the equations to convert a point (x,y,z) to its mappedequivalent (x_(p),y_(p),z_(p)) are:

$x_{p} = {{int}\left( {\frac{x - X_{\min}}{X_{\max} - X_{\min}} \cdot 256} \right)}$$y_{p} = {{int}\left( {\frac{y - Y_{\min}}{Y_{\max} - Y_{\min}} \cdot 256} \right)}$$z_{p} = {{int}\left( {\frac{z - Z_{\min}}{Z_{\max} - Z_{\min}} \cdot 256} \right)}$

For spatial position mapping, the map may contain a three dimensionalarray of sensor readings taken at predetermined intervals. Theseintervals are specified, for example, in three tables, one per axis.Under this arrangement, each table contains a list of normalized (butnot necessarily evenly linearly spaced) axis positions, arranged inascending order. Any combination of X, Y, and Z values taken from thetables thus defines the position of a mapped point and the location of avalue within a table determines an index into the 3D array of sensorreadings.

As an example of extracting a mapped value, assume first, that an x axisposition is normalized to x_(p). Then, the X table is searched to findan entry that satisfies the condition Xi≦xp<Xi+1. This gives the index iinto the map array. The y axis index j and the z axis index k are foundin a similar manner. The mapped point will thus conceptually be within abox having corners at Xi, Xi+1, Yj, Yj+1, Zk, and Zk+1. The sensorthreshold at (xp, yp, zp) may be further estimated, such as by employinga three dimensional interpolation technique using the 8 points of theenclosing box and the corresponding map entries of the points.

Also, in a simplified illustrative example of a threshold tableimplementation, the threshold for a given block 82 may be determined byadjusting a target value by an offset value, which is stored in thethreshold table. For example, the steady state sensor signal can bemeasured at a known point, e.g., at a predetermined root block 82. Thecapacitive sensor 16 is then moved to another block 82, and thesteady-state signal of the capacitive sensor 16 is measured. The storedthreshold value for the current block 82 may be an offset correspondingto the difference between the measured signal at the current blockposition compared to the value of the signal at the root block 82. Thistechnique may be utilized to reduce the amount of storage space requiredto store the various threshold values for each block 82. Otherarrangements may be implemented to store the various threshold values.

According to an aspect of the present invention, the approximateposition of the capacitive sensor 16 (and/or the position of other knownsurface components of the system relative to the capacitive sensor 16)must be known in order to determine which block 82 represents theapproximate current position of the capacitive sensor 16. For example,the controller 38, motion controller 84, or other controller, mayascertain the approximate location of the capacitive sensor 16 in space.The position of the capacitive sensor 16 may be determined for example,using servo position feedback devices such as encoders or resolvers,potentiometers, appropriately placed limit switches and other knownposition determining techniques. The position of the capacitive sensor16 may be computed, e.g., by knowing a previous position andcorresponding position offsets, or the position of the capacitive sensor16 may be approximated, e.g., using heuristics and other processingtechniques. If machine position information can be made available to thecontroller 38, the threshold adjusting procedure could be performedwithin the controller 38 itself, simplifying the interface between themachine 14 and the proximity detection system 10. Otherwise, thecontroller 38 may output suitable information to another machineprocessor for suitable processing.

Machine position information may be unnecessary if other techniques areavailable to determine position, e.g., if the machine moves through apredetermined path or exhibits determinable movements in response todetectable events. Under this arrangement, the threshold table may bebuilt by moving the capacitive sensor 16 through its entire path whileperiodically recording sensor readings. Each reading may become athreshold, such as by subtracting an offset from it. Under thisarrangement, if the capacitive sensor 16 is utilized in a collisionavoidance system, as the capacitive sensor 16 follows its path thesensor readings are compared to the expected capacitance profile that isstored in the threshold table.

As noted above, it is possible that asynchronous as well as synchronousmotions may occur that disturb the field of the capacitive sensor 16where those disturbances are caused by structures that are incapable ofcausing a collision or should otherwise not be detected. For example, arobotic arm may be independently operated in such a manner that it maymove proximate to a capacitive sensor 16 installed on a machine 14. Dueto the physical positioning of the robot arm, it may be possible for therobot arm to get close enough to the capacitive sensor 16 to disrupt thefield and thus affect the measured capacitance, but not close enough tocause an impact with the machine 14. As another example, an arm having acapacitive sensor 16 thereon may be required to reciprocate in and outof an aperture in a housing. The housing around the aperture may disruptthe field of the capacitive sensor 16 thus affecting capacitancemeasurements despite not being capable of causing a collision with thearm.

As such, as described through numerous examples above, the processor isoperatively configured to evaluate a measurement of the circuitincluding the capacitive sensor 16 and to trigger a predetermined actionif the evaluation corresponds to a triggering event, where the processordynamically updates at least one parameter associated with theevaluation. For example, the processor may update a lookup table addressof a threshold value or otherwise modify a threshold value, modifyparameters used to evaluate a function, expression and/or rule etc., toaccommodate changes in at least one system condition. The change insystem condition may comprise, for example, a position of the capacitivesensor 16, a relative time, knowledge of the position of otheranticipated objects near the capacitive sensor 16, etc.

Referring to FIG. 7, another exemplary approach to providing axisposition information to the controller 38, e.g., a system DSP isillustrated. In the arrangement illustrated in FIG. 7, the controller 38is provided with the axis positions directly, thus avoiding anypotential delay that may be required for the motion controller 84 tocalculate the map index k and to send the information corresponding to kto the controller 38. As illustrated, axis position signalscorresponding to the location of the capacitive sensor 16 arecommunicated to an axis interface 86 that couples the position signalinformation directly to the processor/controller 38. The controllercomputes the map index k into the threshold table 82 to obtain thecurrent position.

As noted above, although described with reference to a threshold table82 for purposes of illustration, the comparison that is performed todetermine whether an event of interest has occurred requiring action,e.g., encroachment of an object of interest such as a person or othermachine part towards the capacitive sensor 16, may be based upontechniques other than threshold values. For example, functions such asrules, algorithms, expressions and other types of information may beevaluated, computed or otherwise derived in making such decisions. Forexample, a function that determines a rate of change of capacitance maybe used to evaluate whether an action is required. Under thisarrangement, if a rate of change exceeds a predetermined function value,then an action may be taken.

Referring to FIG. 8, according to another aspect of the invention, ifdefined objects should not be sensed as potential targets of interest,e.g., collision targets, then such structures can be shielded from theproximity detection system 10. For example, if an insulated electrode,driven by the same signal that drives the sensor shield 16 and itscable, is placed between the object and the sensor, then the apparentcapacitance measured by the control electronics 18 can be made todecrease, rather than increase, as the capacitive sensor 16 approachesthe object, thus providing a mechanism for object discrimination. Forexample, the illustrated proximity detection system 10 is substantiallythe same as the proximity detection system 10 illustrated and describedwith reference to FIG. 1. However, as shown, independent shields 88 arepositioned as necessary, e.g., on or around parts of the machine 14 thatare not part of the assembly of the capacitive sensor 16. The shields 88are driven by the same signal that drives the shield electrode 22.

Referring to FIG. 9, a machine 14, illustrated as a coordinatemeasurement machine such as a gear measurement machine, includes a firsthousing member 14A and a second housing member 14B. A first arm 90A isoperable to traverse up and down within the first housing member 14A,e.g., substantially in a vertical plane. A first capacitive sensor 16Ais coupled to the first arm 90A. Similarly, a second arm 90B is operableto traverse up and down within the second housing member 14B, e.g.,substantially in a vertical plane. The second arm 90B is alsocontrollable to transition out from and retract into the second housingmember 14B, e.g., substantially in a horizontal plane. A secondcapacitive sensor 16B is coupled to the second arm 90B. The first andsecond capacitive sensors 16A, 16B are constructed in a manner describedmore fully herein. Moreover, the processing/electronics 18, includingthe controller 38, (not shown) can be placed in any appropriatelocation.

The sensors 16A, 16B may be physically attached to, or integrated intothe construction of their respective first arm 90A and second arm 90B.The face of the first housing member 14A includes a shield 92A that isdriven by the same excitation signal used to drive the shield electrodeof the first capacitive sensor 16A. As described more fully herein, thisprevents the corresponding controller from triggering a false positiveobject signal as the first arm 90A is moved up and down, in the firsthousing member 14A while inspecting the gear 92. Likewise, the face ofthe second housing member 14B includes a shield 92B that is driven bythe same excitation signal used to drive the shield electrode of thesecond capacitive sensor 16B. This prevents the controller fromtriggering a false positive object signal as the second arm 90B is movedup and down, and in and out of the second housing member 14B whileinspecting the gear 92.

In the exemplary machine 14, the work piece comprises a gear 92 that isto be inspected. In order to prevent the gear 92 and the first andsecond arms 90A, 90B from affecting the evaluation of sensormeasurements so as to otherwise cause an unintended triggering event,the proximity detection system may utilize table lookup and/or othertechniques as described more fully herein, for each capacitive sensor16A, 16B, e.g., such as that described with reference to FIGS. 6-7 toproperly filter, compensate for or otherwise tune out, the respectiveeffects of those components.

Referring to FIG. 10, an exemplary approach to utilizing multiplesensors 16 is illustrated. A capacitive sensor 16 is provided for eachmachine component surface where it is desired to provide proximitysensing. For example, as shown, there are N sensors, each associatedwith a machine component surface of interest (1−N). Each capacitivesensor 16 is coupled to the processing electronics 18 as described morefully herein. For example, each capacitive sensor 16 may be coupled to aunique instance of the circuit 34, digital to analog converter 40,signal conditioning circuit 42, buffer/amplifier 48, analog to digitalconverter 50 and driver circuit 54. However, a single DSP or othersuitable processor may perform all of the processing for each of thesensors 16. Moreover, a single physical digital to analog converter 40or analog to digital converter 50 may be utilized, for example, wherethe physical implementation can be divided out, such as by havingmultiple channels or instances of the converter in a single package, orby performing a multiplexing operation, etc.

For example, four parameters may be specified in the processor to set upeach sensor: its excitation frequency, which is programmed to adifferent frequency for each sensor, the number of cycles in the lookuptable that approximates the excitation signal, the filter output samplerate fsr, and the DFT filter length. This approach allows the system tobe expanded and/or modified without substantial changes to source codethat controls the digital signal processing. For example, additionalsensors can be easily added to a system by simply establishing a uniqueset of parameters for the new sensor, without a need to reconfigure theolder sensor parameters.

Using these programmed numbers for each capacitive sensor 16, thecontroller 38 calculates the T length excitation signal look up tableand two coefficient buffers (based upon the programmed DFT filterlength), one for the sine filter, and one for the cosine filter. Duringoperation, the controller 38 outputs an excitation signal to eachcorresponding digital to analog converter 40 based upon its programmedexcitation frequency. The controller 38 also receives samples from eachcorresponding analog to digital converter 50, and processes each samplestream substantially as described more fully herein. Alternatively, moreand/or different parameters may be specified other than those describedabove.

Referring to FIG. 11, the proximity detection system 10 is operableusing any suitable sensor configuration, including sensors 16 that aresubstantially flat, generally, conformal to a corresponding part, etc.For example, in an illustrative arrangement, a sensor 16 is illustratedon a three dimensional (3-D) probe arm tube 90. The sensor 16 comprisesa plurality of layers including an optional adhesive layer (not shown),an inner insulator 94, a first conductive layer 96, an intermediateinsulator 98 and a second conductive layer 100. In particular, the probearm tube 90 is covered with the inner insulator 94. For example, theinner insulator 94 may comprise a heat-shrink tube or other insulatingmaterial(s). The first conductive layer 96 is formed over the innerinsulator 94 to form the shield electrode 22. For example, the innerinsulator 94 may be coated with a layer of electrically conductive paintor other conductive material(s). The intermediate insulator 98 is placedover the first conductive layer 96 and may comprise a heat shrink tubeor other insulating material(s). The second conductive layer 100 isformed over the intermediate insulator 98 to form the sense electrode20. For example, the second conductive layer 100 may also comprise alayer of conductive paint or other conductive material(s). An outerprotective layer 102 may be provided over the second conductive layer100 to provide protection, durability and any desired aesthetics to thesensor 16.

As yet another example, the sensor may comprise one or more generallyflat “adhesive stickers” that are connected in parallel. In theexemplary structure of FIG. 11, the probe arm tube 90 has a generallyrectangular cross-section. As such, the sensor 16 may be comprised offour assemblies, e.g., implemented as generally flat, adhesive stickersarranged such that one adhesive sticker is applied to each of the foursurfaces. Each adhesive sticker comprises an inner insulator 94, a firstconductive layer 96, an intermediate insulator 98, a second conductivelayer 100 and outer protective layer 102 as described above. Moreover,the first conductive layer 96 of each of the stickers is electricallyconnected in parallel and the second conductive layer 98 of each of thestickers is electrically connected in parallel to form a single sensor16.

The dimensions shown in FIG. 11 are exaggerated for purposes of clarityof discussion herein.

As noted above, a capacitive sensor may be attached to, incorporated orintegrated with, or otherwise associated with a position of interestwhere it is desirable to detect, track, monitor or otherwise recognizethe presence of objects that are or become proximate to the position orpositions of interest. Accordingly, capacitive sensors and thecorresponding process electronics can be integrated with machines orretrofitted onto existing machines.

The software aspects of the present invention may be stored, implementedand/or distributed on any suitable computer usable or computer readablemedium(s), including but not limited to, any medium that can contain,store, communicate, propagate or transport the program for use by or inconnection with an instruction execution system of a correspondingprocessing device. The computer program product aspects of the presentinvention may have computer usable or computer readable program codeportions thereof, which are stored together or distributed, eitherspatially or temporally across one or more devices. A computer-usable orcomputer-readable medium may comprise, for example, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, device, or propagation medium.

These computer program instructions may also be stored in acomputer-readable memory that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousaspects of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention.

Having thus described the invention of the present application in detailand by reference to embodiments thereof, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

What is claimed is:
 1. A proximity detection system comprising: anexcitation source that generates an excitation signal; a capacitivesensor having a sense electrode that secures to a surface of a systemcomponent, wherein the capacitive sensor forms a circuit with theexcitation source to generate a capacitive field about the senseelectrode at least in a direction of interest, wherein the capacitivefield changes due to the presence of objects present within thecapacitive field, thus changing the overall capacitance of thecapacitive sensor; a monitor coupled to the circuit formed of theexcitation source and capacitive sensor that obtains measurements thatare influenced by the capacitive field associated with the capacitivesensor; a mapping of threshold values at a plurality of positions of thecapacitive sensor within a space; and a controller that, for a givenposition of the capacitive sensor within the mapped space: determines ameasured value from at least one measurement from the monitor for thegiven position; retrieves, based at least in part upon the givenposition, a threshold value from the mapping to derive an anticipatedvalue; performs an evaluation based upon the determined value and theanticipated value; and performs a predetermined action if the determinedvalue is outside a predetermined range of the anticipated value.
 2. Theproximity detection system according to claim 1, wherein the mapping ofthreshold values is stored in a threshold table.
 3. The proximitydetection system according to claim 2, further comprising a motioncontroller that monitors the position of the capacitive sensor todetermine an index into the threshold table based upon the mapping. 4.The proximity detection system according to claim 2, further comprising:an axis interface that provides coordinates of the position of thecapacitive sensor to the controller to define an index into the mappingof the threshold table.
 5. The proximity detection system according toclaim 2, wherein: the threshold table comprises a plurality of thresholdvalues, each threshold value at a designated position within a definedspace, where each threshold value accounts for interference with thecapacitive field that is anticipated within normal conditions.
 6. Theproximity detection system according to claim 2, wherein the controllerderives the anticipated value by using the retrieved threshold valuefrom the table as an offset parameter.
 7. The proximity detection systemaccording to claim 2, wherein the threshold mappings are stored in amulti-dimensional format such that the threshold value at a givenposition within a defined space changes as a function of time.
 8. Theproximity detection system according to claim 2, wherein the thresholdtable defines a spatial mapping defining at least a three dimensionalarray of sensor readings taken at predetermined intervals; the thresholdtable comprises three separate tables; the intervals are specified inone table per axis, each table containing a list of normalized axispositions; and any combination of three dimensional coordinate valuestaken from the tables thus defines the position of a mapped point andthe location of a value within a table determines an index into thethree dimensional array of sensor readings.
 9. The proximity detectionsystem according to claim 1, wherein the mapping of threshold values isimplemented from a function.
 10. The proximity detection systemaccording to claim 1, further comprising at least one shield that isplaced between the capacitive sensor and a system component that is notintended to trigger the predetermined action, wherein each such shieldis driven by a copy of the excitation signal, wherein the capacitivesensor and the system component move relative to each other.
 11. Theproximity detection system according to claim 1, wherein: the controlleris configured to control, based upon at least one user-assignableparameter, at least one of the excitation source, the mapping ofthreshold values and the performance of the evaluation based upon thedetermined value and the anticipated value.
 12. A proximity detectionsystem comprising: an excitation source that generates an excitationsignal; a capacitive sensor having a sense electrode that secures to asurface of a system component, wherein the capacitive sensor forms acircuit with the excitation source to generate a capacitive field aboutthe sense electrode at least in a direction of interest, wherein thecapacitive field changes due to the presence of objects present withinthe capacitive field, thus changing the overall capacitance of thecapacitive sensor; a monitor coupled to the circuit formed of theexcitation source and capacitive sensor that obtains measurements thatare influenced by the capacitive field associated with the capacitivesensor; a mapping of threshold values at a plurality of points in timerelative to a start time of a programmed operation of the systemcomponent to which the capacitive sensor is attached, each thresholdvalue uniquely associated with the corresponding point in time of thecorresponding programmed operation; and a controller that evaluates thecapacitive field of the capacitive sensor during execution of theprogrammed operation of the system component at points in timecorresponding to the mapping of threshold values, wherein thecontroller, for a given point in time of the mapping: determines ameasured value from at least one measurement from the monitor; retrievesa threshold value from the mapping corresponding to the given point intime of the mapping, to derive an anticipated value; performs anevaluation based upon the determined value and the anticipated value;and performs a predetermined action if the determined value is outside apredetermined range of the anticipated value.
 13. The proximitydetection system according to claim 12, wherein the mapping of thresholdvalues is stored in a threshold table.
 14. The proximity detectionsystem according to claim 13, further comprising a motion controllerthat monitors the position of the capacitive sensor over time todetermine an index into the threshold table based upon the mapping. 15.The proximity detection system according to claim 13, wherein: themapping of threshold values further comprises a plurality of thresholdvalues, each threshold value at a designated position within a definedspace, where each threshold value accounts for interference with thecapacitive field that is anticipated within normal conditions.
 16. Theproximity detection system according to claim 13, wherein the controllerderives the anticipated value by using the retrieved threshold valuefrom the table as an offset parameter.
 17. The proximity detectionsystem according to claim 12, wherein the mapping of threshold values isimplemented from a function.
 18. The proximity detection systemaccording to claim 12, wherein the threshold mappings are stored in amulti-dimensional format such that the threshold value at a given pointin time also corresponds to a position within a defined space.
 19. Theproximity detection system according to claim 12, further comprising atleast one shield that is placed between the capacitive sensor and asystem component that is not intended to trigger the predeterminedaction, wherein each such shield is driven by a copy of the excitationsignal, wherein the capacitive sensor and the system component moverelative to each other.
 20. The proximity detection system according toclaim 12, wherein: the controller is configured to control, based uponat least one user-assignable parameter, at least one of the excitationsource, the mapping of threshold values and the evaluation of thecapacitive field of the capacitive sensor during execution of theprogrammed operation.